Details needed here I think. What are the settings of your project and properties of your video?
I've never had this. You may have simply imported a large video into a project of smaller dimensions and PPro has simply cropped the vid to 'fit'. i.e. A PAL video imported into an NTSC project. PAL video is a bit 'bigger' that NTSC so you'll need to shrink it down to fit or crop it.